The scanning tunnelling microscope (STM) has contributed to the development of new micro characterisation processes which are based on the use of a very fine point. One such process is atomic force microscopy (AFM).
In the original design of AFM, described in G. Binnig, C. F Quate, C. Gerber, 1986, Atomic Force Microscope, Phys. Rev. Lett. 56, 930-933 and in EP-A-O 223 918, a sensor consisting of a spring-like beam which is rigidly fixed at one end and has a point at the other free end samples the surface of an object line by line. The deflection occasioned by the force between the surface of the object and the point can be measured with the greatest degree of accuracy.
Up until now, there have basically been two types of point used: conical points and countersunk points as described in European Patents EP-A-O 413 042, EP-A-O 413 041, EP-A-O 413 040 and EP-A-O 468 071, for example, as well as in the IBM Technical Disclosure Bulletin, vol. 37, No. 07, July 1994, pages 545-546.
FIG. 2a shows a conical point. The subjects for measurement using conical points are mainly surface roughness and step heights.
Countersunk heads as in FIG. 2b are particularly suitable for measuring structures with vertical flanks in order to measure the width, depth, flank angles or roughness of the side walls.
In most cases, only one material is used in the manufacture of points, this mainly being single-crystal silicon. The points described in EP-A-O 413 042 are constructed from thinly deposited films, e.g. from silicon nitride or silicon carbide. Single-crystal silicon has been shown to be particularly advantageous for working and shaping points. Countersunk points, in particular, with their requirements regarding height, diameter and overhang can only be manufactured at acceptable cost from this material.
In general, for the points used for measurement, the diameter of the point must be smaller than the structure to be measured. The diameter of the shank of a countersunk point is then smaller again than the diameter of the head of the point. The length and diameter of the vertical point shank determine the mechanical lateral rigidity of the point for stated materials.
If the rigidity is insufficient, then the point will adhere or stick to the side wall of the structure being measured during sampling, as shown in FIG. 3. The point can then only be removed from the side wall with difficulty. Even the distortion of the point shank as a result of attractive van der Waals forces between the point and the side wall can lead to erroneous measurements. In addition, particularly small countersunk heads are susceptible to damage during normal use.